1. Field of the Invention
The present invention relates to batteries containing an alkaline electrolyte, hereinafter referred to as alkaline batteries, in which the negative electrode is constituted of either zinc or cadmium and the positive electrode of either silver oxide or nickel.
2. Prior Art
Of the different materials suitable for use in an electrochemical cell, zinc would seem particularly attractive, owing to its high energy capacity and low cost. However, two disadvantages arise after a number of charging and discharging cycles, using the zinc electrode in an alkaline electrolyte. Firstly, there is a decantation of the reaction products as a result of the formation, during the discharge process, of complex soluble anions, known as zincate ions, which accumulate at the bottom of the battery owing to the force of gravity. During the recharging period, a deposit tends to accumulate at the lower part of the electrode and hydrogen is given off at the upper part. During successive cycles, the zinc accumulates at the lower part of the electrode to the detriment of the upper part of the electrode. This disadvantage may be partially eliminated by using a separator, i.e., a membrane permeable to the electrolyte and impermeable to the zincate ions. This membrane is situated between the electrodes.
Secondly, during the recharging process, the zinc deposited on the negative electrode tends to form points, known as dendrites. This leads to the perforation of the separator and causes the battery to short-circuit.
In the case of cadmium-nickel batteries, a troublesome phenomenon tends to occur -- oxygen is given off at the positive electrode. This discharge of oxygen stops the charging of the negative electrode due to the oxygen migrating towards it.
As a result, the chances of recharging the battery are considerably reduced after a relatively short period of time.
In the case of batteries containing a zinc electrode, the above-mentioned disadvantages may be eliminated by using a separator, which is selective with respect to the zincate ions and which responds satisfactorily both mechanically and chemically. It is important that the separator should be sufficiently reliable and that it should not be affected by the inevitable presence of oxygen on the positive electrode (silver oxide or nickel electrode) at the end of the recharging process.
At present, the most commonly used separator is cellophane. This material, which is attractive because of its extremely low cost price, does not exhibit a good selective action with respect to the zincate ions. On the other hand, its reliability is limited owing to its susceptibility to deterioration by oxygen.
Separators consisting of a polyamide felt and cellulose have also been proposed in connection with cadmium-nickel batteries. However, cellulose does not hold up well in an alkaline electrolyte and it is very much affected by the presence of oxygen.